Gustav Sohlenius

AUTHIGENIC SULFIDE AND CARBONATE MINERAL FORMATION IN HOLOCENE SEDIMENTS OF THE BALTIC SEA

Formation of authigenic sulfides and carbonates in Holocene 14C-dated sediments is interpreted based on analyses of cations, FeSS, FeS2S, Corg and sulfur isotopes. Mineral distribution was studied in thin sections and mineral morphology with SEM-EDS. The sediments were deposited in the Gotland Deep, Baltic Sea, during freshwater and brackish stages. Calcium-rich rhodochrosite (MnCO3) occurs in concentrations up to 17%dw in brackish water sediments and are mainly confined to 0.02–0.5-mm-thick laminae. The frequency with which these laminae occur suggests that they formed with a periodicity of one to a few years, and their purity suggests that formation took place in a few weeks. We propose that these carbonate laminae resulted from oxic water inflow to the euxinic basin. Mn(II) dissolved in the water column was rapidly oxidized and large amounts of Mn-oxides reached the sediment. Reduction of these Mn-oxides in a high-alkalinity environment facilitated rhodochrosite precipitation at the sediment surface. Pyrite in the brackish water sediments formed at or above the sediment-water interface, in an iron-limited environment. Due to alternating oxic and euxinic conditions FeS was oxidized, resulting in a high FeS2SFeSS ratio (median = 170). Formation of iron sulfides in the freshwater sediments occurred below the sediment-water interface, in a sulfate-limited environment. As a result of the rapid exhaustion of reactive iron in the brackish water sediments, excess H2S diffused downwards to pyritize the uppermost part of the underlying freshwater sediments.

Holocene history of the Baltic Sea as recorded in a sediment core from the Gotland Deep

A 4 m long sediment core from the Gotland Deep, Baltic Sea, was investigated by means of biostratigraphical and chemical parameters and C-14 datings. This multidisciplinary approach allows us to ev ...

Development of anoxia during the Holocene fresh-brackish water transition in the Baltic Sea

One of the most pronounced environmental changes during the Holocene Baltic Sea history was the transition from the freshwater Ancylus Lake to the brackish water Litorina Sea. The establishment of ...

Diatom stratigraphy in five offshore sediment cores from the northwestern Baltic proper implying large scale circulation changes during the last 8500 years

Five Holocene sediment cores from the northwestern Baltic proper were analysed for lithology, siliceous microfossil assemblages and geochemical parameters. The data indicate that surface water salinity and redox conditions below the halocline have changed drastically at least four times since the Baltic Sea changed from a fresh water lake (the Ancylus Lake) to a semi-enclosed brackish water sea (the Litorina Sea) c. 8500 yrs BP. These variations appear to be mainly effects of changes in water depth at the thresholds of inlet areas. Based on these changes, and earlier studies of the shoreline displacement in the inlet areas, we propose a tentative model for changes of large scale water circulation in the Baltic Sea during the last c. 8500 yrs.At the transition from fresh to brackish water 8500 14C yrs BP, upwelling of nutrient rich bottom water started to occur, causing a slight increase in primary production. Diatom assemblages in sediments indicate a slow rise in surface water salinity during this period.At 7000-6500 14C yrs BP, surface water salinity and primary production simultaneously increased, as anoxic bottom conditions were established at depth below the halocline. We suggest that high primary production was caused by increased input of oceanic water, leading to increased upwelling of nutrient rich bottom water. At the anoxic bottoms laminated sediments formed until 5000-4500 14C yrs BP. This period (c. 7000-4500 14C BP) was contemporaneous with the post-glacial transgression maximum in Öresund, and we suggest it represents the most saline phase of the Baltic Sea post-glacial history.Due to a regression in Öresund starting 4500 14C yrs BP, upwelling decreased and the halocline was lowered, resulting in decreased primary productivity and hence oxic deep water conditions. The diatom assemblages of the sediments indicate a lowering of salinity at the beginning of this period.We suggest that the second period of anoxic bottom conditions c. 2000-1500 14C yrs BP was caused by a change of dominating inflows from the Öresund to the Belt Sea. This resulted in decreased salinity of the inflowing water which did not penetrate to the deepest parts of the basin as frequently as before. The diatom record indicates both a second lowering of salinity and a change in the large scale water circulation at the beginning of this period.

Different phases of the Yoldia Sea in the north-western Baltic Proper

Four sediment cores from the north-western Baltic Proper, covering the time from the Weichselian deglaciation to the present have been studied. Main interest is focused on the Yoldia stage, and especially the duration of the marine phase of this stage. The lithological compositions of the sediments and results of different analyses (magnetic, carbonate, ostracods and foraminifera) have resulted in a subdivision into five stratigraphical units (A-E, from older to younger). Unit A was deposited in a glaciolacustrine environment, probably during the first non-saline phase of the Yoldia stage. The first ingression of saline water is recorded in unit B. Brackish water ostracods and foraminifera occur exclusively in this unit, which represents the only saline phase of the Yoldia stage. This phase lasted for less than 120 clay varve years. Unit C was deposited in fresh water, probably during the last phase of the Yoldia stage and the Ancylus stage. The uppermost units, D and E are of Holocene age and represent different phases of the Litorina and Post-Litorina stages.

Sedimentary Trace Elements as Proxies to Depositional Changes Induced by a Holocene Fresh-Brackish Water Transition

A halocline developed in the GotlandDeep, Baltic Sea, at c. 8.0 14C ky BP, as theresult of a transition from fresh to brackish water.The sediment-water interface changed from oxic topredominantly anoxic, depositional conditions wereperiodically euxinic and pyrite formation wasextensive. This environmental change led topyritization of the upper part of earlier depositedsediments. This study discusses how the distributionof trace elements (As, Ba, Cd, Cu, Co, Mo, Mn, Ni, Pb,U, Zn and V) were affected by the changing redoxconditions, productivity and salinity. The reducingconditions led to pyritization of Cu, Co, Ni, Cd, Mo,Mn and As. Lead and Zn concentrations are very low inpyrite, in agreement with their coordination tosulfide being tetrahedral. Certain elements areenriched in those sediments deposited under euxinicconditions. This enrichment was caused by scavengingof elements dissolved in the water column and isrestricted to elements that have a comparably longresidence time in the Baltic Sea. Molybdenum appearsto be the most unambigious proxy for euxinicconditions, whereas enrichment of U also requiresbrackish water in the productive zone. In the brackishenvironment, enrichment of Ba and V are linked to thecycling of organic carbon. Manganese and As are theonly elements that have been significantly remobiliseddue to the downward moving pyritization front.

Evidence of benthic colonisation during formation of laminated sediments in the Gotland Deep, Baltic Sea

Abstract Benthic foraminifera were found in laminated non-bioturbated sediments from the Gotland Deep in the Baltic Sea. The sediments were deposited during the Littorina Sea stage (8000–3000 14C yr. BP), where euxinic conditions (anoxic bottom water) prevailed in the deepest part of the basin. The bottom water became occasionally oxygenated when saline oxic water entered the basin. During some of the oxic periods, a benthic oxygen-demanding fauna was able to colonise. The oxygen levels below the sediment-water interface were, however, too low to admit a burrowing fauna. The foraminifera were protected from upward diffusing H2S by a layer of manganese oxide acting as a redox buffer. It is concluded that colonisation of benthic foraminifera occurred during short periods suggesting that an establishment could take place if the deep bottoms of the present-day Baltic Sea became oxic.

The isolation age and history of Lake Sågsjön, Stockholm, based on different dating techniques

Abstract Laminated sediments from Lake Sågsjön, situated east of Stockholm, were used to study the last 1000 years history of the lake. A combination of 14C dated (AMS) terrestrial macrofossils and annually laminated sediments were used, together with diatom stratigraphy, to determine the age of the isolation from the Baltic Sea. The laminae were counted using greyscale analysis and studied in detail by thin sections. Three new pair of laminae were formed between 1993 and 1996, indicating that the laminae are annually formed as varves. This is further supported by studies of thin sections, which reveal organic and minerogenic layers reflecting summer algae bloom and spring flood, respectively. Two of the 14C dated macrofossils are interpreted as redeposited wheras the calibrated ages received from two leaf fragments are believed to represent the age of sediment deposition. According to the varve counting, Lake Sågsjön (2.0 m a.s.1.) was isolated from the Baltic Sea between AD 1500 and 1600. The two radiocarbon dated leafs give an isolation age between AD 1150 and 1400. Compared to the 14C dates, the laminae based chronology yields too young ages, which indicates that the deposition of the laminae was not continuous. Shortly after the isolation the rate of sediment accumulation increases, which most probably is an effect of increased erosion when the catchment area was partly brought under cultivation.